Reports of heavy periods – bleeding characterised by increased volume and/or duration that interferes with the quality of life – have been observed during clinical trials, from cases in the real world and in medical literature, the EMA said.

The cases – which have mostly been non-serious and temporary in nature – have been reported after the first, second and booster doses of Pfizer-BioNTech’s Comirnaty and Moderna’s Spikevax, it added.

The regulator has now concluded that there is at least a “reasonable possibility” that heavy menstrual bleeding is causally associated with these vaccines.

Pfizer, BioNTech and Moderna were not immediately available for comment.

The EMA also said that there was no evidence to suggest that the menstrual disorders experienced by some have any impact on reproduction and fertility.

The mRNA vaccines do not cause pregnancy complications for expectant mothers or their babies and they are as effective at reducing the risk of hospitalisation and deaths in pregnant people as they are in non-pregnant people, a review carried out by the EMA’s Emergency Task Force has shown.

Overall, the totality of data continues to show that the benefits of these vaccines greatly outweigh the risks, the agency reiterated today.

Menstrual disorders can occur due to a range of reasons, including underlying medical conditions as well as stress and tiredness. Health authorities have highlighted that cases have also been reported following Covid-19 infection.